where to start
all of the roms on the artemis part of this site should work with the orbit, but you'll need to USPL first.
None of them are o2 branded because there isnt an official o2 rom, but you can add the various settings you'll need manually afterwards. An o2 rom would (if it was released) effectively be the same as one of these roms but with a different wallpaper etc.
Just upgraded my o2 orbit using black & blue 5 off this site and after USPL-ing it it went fine.
use the search on this site for any problems you've got, and once you find a rom you like the look of read through the ENTIRE thread to see what (if any) problems other people have had with it and what (if any) solutions were found
only install a rom suitable for the ARTEMIS though (you're on the right page)